Fix shutdown error.
Fix shutdown error.

None type can be in skill list

Dark5ide/mycroft-core,MycroftAI/mycroft-core,aatchison/mycroft-core,linuxipho/mycroft-core,aatchison/mycroft-core,Dark5ide/mycroft-core,linuxipho/mycroft-core,forslund/mycroft-core,forslund/mycroft-core,MycroftAI/mycroft-core